The present disclosure relates generally to preemption in a computer network.
The IETF (Internet Engineering Task Force) is investigating the area of stateless or reduced-state admission control for real-time inelastic traffic. (See, B. Briscoe et al., “An edge-to-edge Deployment Model for Pre-Congestion Notification Admission Control over a DiffServ Region”, IETF draft-briscoe-tsvwg-cl-architecture-04.txt, Oct. 25, 2006 and A. Bader, “RMD-QOSM—The Resource Management in Diffserv QOS Model”, draft-ietf-nsis-rmd-07.txt, Jun. 23, 2006). One of the challenges related to stateless (as well as stateful) Call Admission Control (CAC) is that occasional topology changes (e.g., in response to a link or node failure) may result in bypassing the CAC completely as routing redirects the traffic to a different path than it was admitted to originally. This can lead to severe congestion that could last for a potentially long time. In these circumstances, links which normally operate under their engineered load due to admission control become suddenly overloaded. As a result, all real-time flows sharing a congested link can become affected and suffer possibly severe QoS (Quality of Service) degradation. If these flows are voice flows, for example, then such QoS degradation may result in all, or many users on the link eventually hanging up and dropping their connection if substantial service degradation lasts longer than a few seconds. It is, therefore, desirable to have a mechanism which will selectively preempt some number of flows to alleviate congestion, restoring the necessary level of QoS to the other flows. This mechanism is referred to as Preemption or Severe Congestion Handling in the above-referenced IETF drafts. It is desirable that such preemption is done on a per-flow basis rather than per-packet as much as possible, to prevent a degradation of service to all flows involved in congestion.
Similar problems may arise not only with networks running an admission control mechanism, but also in networks that use bandwidth provisioning based on the knowledge of the traffic matrix. For example, if real time traffic is using EF PHB (Expedited Forwarding Per-Hop Behavior) and the network is properly provisioned in the absence of failure as well as under some expected failure scenario, an unexpected link, node or SRLG (Shared Risk Link Group) failure or an unexpected combination of those may result in a sudden overload of some links, causing QoS service degradation. Preemption is also useful in networks which do run admission control in cases where there is an unexpected traffic surge.
Hence, regardless of the presence or lack of bandwidth reservations under normal, non-failure conditions, the possibility of failures (without full bandwidth protection of those failures as is commonly the case in packet networks) results in a need for a mechanism that allows preemption of a set of flows while allowing the rest of the flows to maintain their QoS guarantee. There are two solutions that have been proposed in the above-referenced IETF drafts. However, both of these solutions require a per-ingress-egress estimate of congestion at the egress edge to determine the correct amount of traffic to drop, and policing of these schemes presents a serious challenge.
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